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What is Anaerobic Digestion?

Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels. A range of anaerobic digestion technologies are converting livestock manure, municipal wastewater solids, food waste, high strength industrial wastewater and residuals, fats, oils and grease (FOG), and various other organic waste streams into biogas, 24 hours a day, 7 days a week. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland or converted into other products. Nutrients in the liquid stream are used in agriculture as fertilizer.

Generic digestion processThe Biological Process
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.

Digester Technologies
Many different anaerobic digester systems are commercially available. The following is an overview based on organic waste stream type (manure, municipal wastewater treatment, industrial wastewater treatment and municipal solid waste): Manure: Anaerobic digestion systems for livestock manure operate to reduce methane emissions, odors, pathogens and weed seeds and produce biogas. They fall into four general categories:
  1. Covered anaerobic lagoon digester: Sealed with flexible cover, with methane recovered and piped to the combustion device. Some systems use a single cell for combined digestion and storage.

  2. Plug flow digester: Long, narrow concrete tank with a rigid or flexible cover. The tank is built partially or fully below grade to limit the demand for supplemental heat. Plug flow digesters are used at dairy operations that collect manure by scraping.

  3. Complete mix digester: Enclosed, heated tank with a mechanical, hydraulic, or gas mixing system. Complete mix digesters work best when there is some dilution of the excreted manure with water (e.g., milking center wastewater).

  4. Dry Digestion: Upright, silo-style digesters made of concrete and steel with rigid cover. Dry digesters operate at 20 to 42 percent total solids, which allows them to combine high dry matter manure and crop residuals with very dilute liquid manures or co-substrates.
Source: http://www.epa.gov/agstar/anaerobic/ad101/anaerobic-digesters.html (diagrams available)

Municipal Wastewater
Wastewater treatment plants employ anaerobic digesters to break down sewage sludge and eliminate pathogens in wastewater. Technologies available for municipal wastewater fall into tthree general categories — mesophilic, thermophilic, and temperature-phased systems.

Industrial Wastewater
Food and beverage manufacturing facilities typically generate high strength waste streams as a by-product of their manufacturing operations. These waste streams are characterized by high Chemical Oxygen Demand (COD) and solids loading, making them well-suited for treatment using anaerobic processes.

Municipal Solid Waste (MSW)
Anaerobic digestion of the organic fraction of MSW provides an engineered and highly controlled process of capturing methane, especially when compared to landfill gas capture of methane generated by putrescible waste. Typically, digestion of mixed solid waste is done as part of compliance with directives to stabilize the organic fraction of the waste stream prior to disposal. The current trend is toward anaerobic digestion of source separated organic waste streams, including food waste, yard trimmings and soiled paper. Presorting is necessary to prevent clogging of the pumps and to reduce the amount of reactor volume occupied by inert material. Even source-separated waste inevitably contains metal and plastic contaminants and must be presorted. Anaerobic digestion systems for MSW include:
  • Single-stage wet digesters: Typically simpler to design, build, and operate and generally less expensive, the organic loading rate (OLR) of single-stage digesters is limited by the ability of methanogenic organisms to tolerate the sudden decline in pH that results from rapid acid production during hydrolysis.

  • Dry fermentation: Type of single-stage digester, but distinctive from other AD categories because feedstocks are in a solid state that can be handled with a front-end loader and normally no additional water is added. Digestion takes place at 20-45% total solids, and can be done in either a batch or continuous mode. In batch mode, materials are loaded into chambers then inoculated and maintained until the end of the retention time. In continuous mode, fresh feedstock is continuously fed to the digester and digestate is continuously removed.

  • Two-stage digesters: System separates the initial hydrolysis and acid-producing fermentation from methanogenesis, which allows for higher loading rates for high nitrogen containing materials but requires additional reactors and handling systems. Another important design parameter is the total solids (TS) concentration in the reactor, expressed as a fraction of the wet mass of the prepared feedstock. The remainder of the wet mass is water by definition. Feedstock is typically diluted with process water to achieve the desirable solids content during the preparation stages.
Source: http://ww.calrecycle.ca.gov/publications/Organics/2008011.pdf

biogas processingHow Biogas is Upgraded to Biomethane/Renewable Natural Gas
Captured biogas is transported via pipe from the digester, either directly to a gas use device or to a gas treatment system (e.g. for moisture or hydrogen sulfide removal). If the feedstocks contains high concentrations of sulfur and no steps are taken to prevent formation of hydrogen sulfide in the digester, then hydrogen sulfide is removed from the biogas to prevent corrosion of the combustion device or other downstream equipment.

Captured biogas can also be further upgraded by removing carbon dioxide, nitrogen and oxygen in order to meet the high purity and BTU requirements for pipeline injection or compressed biomethane vehicle fuel.

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